1. Field of the Invention
This invention relates to release surfaces useful for release liners and adhesive contacting release surfaces for self-wound tapes among other applications. More specifically, the invention combines the use of an epoxypolyorganosiloxane with a crosslinkable silicone hydride resin to yield a material which demonstrates superior release properties.
2. Technology Description
A major utility of the invention concerns an improved release liner (or backing) for use in combination with a pressure-sensitive adhesive layer and a face stock preferably for label applications. In such combinations, the release liner protects the pressure-sensitive adhesive (PSA) prior to the label being used and is removed immediately prior to application of the label to another surface.
Additionally, the release liner serves to facilitate cost effective manufacture of rolls or sheets of labels. It also functions as a carrier of labels for dispensing in automatic labeling operations and for computer printing in EDP applications. The performance attributes of a release liner are critical to both the manufacture and end-use application of pressure sensitive adhesive labels.
In conventional practice, the release liner is provided as a silicone layer on a support layer having high holdout, i.e., the support layer on which the silicone layer is deposited is resistant to silicone penetration. Where the support layer is paper, a special and, therefore, expensive paper, such as a super-calendered or densified glossy paper, is required. One currently accepted way of applying a silicone release composition to a high-holdout support layer is by solvent coating. Growing concern over the environment has imposed stringent restrictions regarding recovery of the solvent used in applying the solvent based silicone to the high-holdout backing paper or other materials.
An alternative to this is to use 100% solids silicone release compositions. These are supplied with a viscosity (usually &lt;2000 cps) suitable for roll-coating techniques. Application of these to porous substrates such as low cost papers, machine finished (MF) or machine glazed (MG) papers, finds these materials to soak into the paper (penetrate the paper surface) to give ineffective coverage of the paper fibers unless excessively high quantities of expensive silicone are used. Ineffective coverage of the paper fibers provides unsuitable release liners for PSA applications especially where high speed convertibility is an essential performance feature.
A major application for a release liner is as part of bulk rolls of laminate consisting of the release liner, a face stock between which there is contained an inherently tacky self-adhesive, or pressure-sensitive adhesive. The adhesive may be permanent or repositionable. The rolls are converted by printing label information on the face stock, die cutting the labels through the face stock and adhesive to the surface of the release liner, followed by removal of the matrix surrounding the labels thus leaving a plurality of labels on the release liner.
It is important that the force required for release be sufficiently low for the intended application, but not so low that the die cut labels will release or predispense from a moving web turning a corner or remove with the waste matrix during its removal. The release force should also be not so high that the matrix is broken during its removal.
It is known to this art to prepare antiadhesive coating compositions for use in the above-described technical fields based on a polyorganosiloxane containing functional groups (of the epoxy or vinyl ether type, and the like) to which a polymerization initiator, such as an onium salt is added for the crosslinking thereof. Examples of patent literature which disclose such coating compositions are in contained in U.S. Pat. Nos. 4,313,988; 4,450,360; 4,576,999; and 4,640,967.
Of particular interest are antiadhesive coating compositions based upon epoxy-functional polysiloxane chemistry because of their ease of use and ability to crosslink and form an excellent release surface when exposed to ultraviolet radiation. Irradiation crosslinking technology has decisive advantages when compared with conventional thermal based systems such as very rapid cure rates, the absence of solvents, the possibility of operating at ambient temperature and therefore on heat sensitive substrates, and the use of minimal maintenance coating machines leading to reduced operating costs. In addition, these materials are capable of being used with reduced toxicity and excellent long term stability of release properties.
The use of various control release modifiers for silicone polymers are disclosed in the following patent publications: U.S. Pat. No. 5,310,601; U.S. Pat. No. 5,158,991; U.S. Pat. No. 5,468,828; U.S. Pat. No. 5,468,816; and EP 0108208. The modifiers are alleged to assist in the functional properties of the final coating compositions. Typically what has been proposed is the utilize a modifier which has a similar functionality to the silicone polymer. For example, in the field of vinyl silicone polymers, the release modifiers suggested have been so called MQ resins which have been modified to have a vinyl functionality. This is suggested in EP 0108208. In the field of epoxypolyorganosiloxane polymers, the release modifiers suggested have had an epoxy functionality. For example, U.S. Pat. Nos. 5,310,601 and 5,158,991 include modifiers which contain epoxy functional organic radicals containing between about 2 and about 20 carbon atoms. The literature fails to disclose or suggest the use of modifiers that do not have functionality correlating to the functionality of the base polymer.
Despite the above advanced associated with release coatings, and in particular those based upon epoxy-functional polysiloxane chemistry, there still exists a need in the art for products having even better release properties than previously expected. The invention at hand utilizes an additional monomer having no epoxy functionality, present in combination with the epoxy-functional siloxane to yield a resulting composition, which, when cured, yields outstanding release properties.